The invention is directed to a fast, contact-free, geometrical as well as thermal characterization of a planar multi-layer structure. Measurements with respect thereto are in demand, for example, in automotive multi-coat lacquering. The category of thermal wave measuring methods are known, for example, under the designations heat sources, photothermal and photoacoustic methods or lock-in thermography.
Methods that, for example, pass by the name xe2x80x9cphotothermal measuring methods, thermal wave measuring methods or lock-in thermographyxe2x80x9d belong to the Prior Art. Therein, a material to be tested and having a superficial layer structure is heated periodically and in regions with a heat source. The heating must be capable of being modulated, so that an amplitude modulation is present. The modulation frequencies of the heating can thus be sequentially tuned, and the photothermal signal that derives from a specimen is measured as a function of the frequency based on amplitude and, in particular, its phase. The evaluation in terms of two or more unknowns (for example, layer thicknesses) can thereby generally not be implemented in closed analytical form since an xe2x80x9cinverse problemxe2x80x9d is present here. This is equivalent to saying that the solving of the equation system for an unknown is not possible without further ado.
Disadvantages of the methods belonging to the Prior Art are comprised, for example, therein that the sequential tuning of the modulation frequency of the modulatable heat source lasts a long time.
The invention is based on the object of offering a thermal wave measuring method with which a significant speed-up of a corresponding measurement and evaluation can be achieved. A critical goal is comprised in the use of a fast thermal wave measuring method for monitoring layering structures in ongoing production.
This object is achieved by the feature combination of claim 1.
The invention is based on the perception that the heat source employed for the regional heating of a layer structure can be simultaneously driven with a plurality of different frequencies and the infrared radiation corresponding to the drive frequencies can be simultaneously evaluated. Thus, specific supporting points can be determined from a characteristic for the sequential tuning of the heat source over the frequency, a specific plurality of different, discrete frequencies deriving therefrom. These are simultaneously employed for the drive of the heat source, so that the actual tuning of the heat source over the frequency is no longer implemented, a significant time-savings deriving therefrom.
Further developments can be derived from the subclaims.
In particular, a light-emitting diode (LED) or a laser diode can be advantageously utilized as heat source since they can be electrically amplitude-modulated. Fundamentally, all heat sources can be utilized that offer the possibility of an electrical modulation such that a multi-frequency excitation can be implemented.
When a specific layer sequence is present at the surface of a specimen, then a subject-related setting of the drive frequencies can be advantageously undertaken at the heat source. The relationship applies that an increasing penetration depth into the layer structure accompanies dropping modulation frequency at the heat source. The selection of the drive frequencies can be advantageously set in conformity with a known layer structure.
The target quantities, for example individual layer thicknesses, can be numerically determined with the approach of a regression analysis with non-linear formulation functions or, respectively, with a trainable neural network. Experimental or theoretical/analytical supporting values can thereby be employed as calibration values.
Further advantageous developments can be derived from the subclaims.